1. Field of the Invention
The present invention relates to a thermal printing system, and more particularly, to a thermal printing system which prints alphanumerical and/or graphic information, on a paper sheet cut into unified sheet size of A4, B5 or the like fed from a cassette case, by melting a film of printing ink applied on a plastic membrane by the heat imparted by a thermal head, which further comprises at least a means for selectively depressing or releasing a pair of pinch rollers against a platen, thereby allowing the user who operates the system to readily remove any paper crumpled between the platen and the pinch rollers.
2. Description of the Prior Art
Various types of thermal printing systems have been developed and are being widely used, which print a number of dots of printing ink corresponding to alphanumerical and/or graphic information printed by a thermal head of the printing system on a paper sheet, cut into unified size, by thermally melting a film of printing ink applied on the surface of a plastic membrane in a dried state at an ambient temperature. In these types of thermal printing systems, paper sheets for printing are supplied as a set of cut paper sheets each cut into such a unified size of A4, B5 or the like and are contained in cassette case each capable of receiving a number of paper sheets, say, 50 or 100, and these paper sheets are automatically fed or propelled as printing proceeds on. That is, a subsequent sheet in advancecd as soon as the preceeding paper sheet has finished being printed on, and the thus newly fed paper sheet is caught, in other words, squeezed between a front pinch roller and a rear pinch roller both of which are disposed as a pair, and the thus caught paper is depressed upon the surface of a platen and is subjected to printing. Through such type of printing systems are designed and manufactured such that the way of feeding successive paper sheets one after another from a paper cassette case to the pair of pinch roller is done automatically, sometimes such trouble may arise that the automatically feeding of the paper sheets becomes unsatisfactory and results in malfunction of the system such as crumpling of a fed paper sheet between the pinch roller or rollers and a platen, due to slight misalignment and/or minor difference in size between the paper sheets, and to a minor deviation in the dimension of parts or components of the system.
By taking these drawbacks in the prior art printing system into account, there has been conceived and developed such a device capable of removing a paper sheet which has actually been crumpled between the pair of pinch rollers as disclosed by Japanese Utility Model Application No. Sho 61(1986)-73966(Unexamined U. M. Publication No. Sho 62(1987)-187750).
This publication discloses a thermal printing system which comprises, as shown in FIG. 8, a platen 21, a pair of pinch rollers 23 each pinch roller of which is disposed to be freely rotatable on each of a pair of pinch levers 22 and capable of being depressed against or released from said platen 21. The aforesaid depressing and releasing action of this printing system is effected in such manner that the depression of the pinch rollers 23 against the platen 21 is done by a pinch roller lever tensioning spring 24 connected between both the swingable ends of the pinch roller levers 22 and the releasing of the depressed pinch rollers is effected by a manual releasing lever 25 rotatable about a pin as an axis of rotation.
This manual release level 25 is provided, at one end, with a rotary disc 26 which normally does not impart any force to the pinch roller levers 22, but acts to make the pinch rollers 23 move away from the platen 21, only when the manual release lever 25 is turned at a specified angle in a direction opposite to that for depressing the pinch rollers, namely, the pinch roller lever at the left side is turned clockwise and the pinch roller lever at the right side is turned opposite, that is, anticlockwise.
The publication also discloses, as another embodiment shown in FIG. 9, a thermal printing system provided with a depression/release lever 25' fixedly attached with a return lever 29 for returning aforesaid release lever 25', in which the return lever 29 acts to return the depression/release lever 25' when the return lever 29 contacts the lower frame 28 when the upper frame 27 is lowered onto the lower frame 28 for closing.
However, according to the thermal printing system dislosed by the above mentioned unexamined U. M. Publication No. Sho 62-187750, it is required for one who uses the system to manually operate the releasing lever 25, it becomes necessary for him to release the pinch rollers 23 in depressed state against the platen 21, so a person who is not skilled in operation cannot know the way of handling the depression/releasing lever unless he reads an operating manual of the device, or he is apt to involuntarily try to pull out the paper sheet P in a crumpled state without manipulating the releasing lever.